This compound, also known by the abbreviation PFOB, is used in numerous fields, in particular in medicine as a radiopaque agent (contrast agent for X-rays) or as an oxygen carrier in blood substitutes.
In its Patent EP 0,298,870 and its Patent Application EP 90403118.4, the Applicant Company described processes for preparing perfluoroalkyl bromides R.sub.F Br (in particular PFOB) from the corresponding perfluoroalkanesulphonyl chlorides R.sub.F SO.sub.2 Cl which are reacted either with gaseous HBr in the presence of a catalyst (EP 0,298,870), or with a quaternary ammonium or phosphonium bromide (EP 90403118.4). The yields obtained are generally high, but the sulphonyl chloride R.sub.F SO.sub.2 Cl used is an advanced material since its synthesis from the corresponding iodide R.sub.F I requires two reaction stages according to the equation: ##STR1## available in industrial quantities. Although this way is more direct, it has the drawback of leading to a product which still contains residual R.sub.F I which it is difficult to separate from the R.sub.F Br desired. This is more particularly the case for PFOB which must have a C.sub.8 F.sub.17 I content less than 100 ppm for its medical applications.
A technique which has been known for a long time and is used industrially for preparation of trifluoromethyl bromide CF.sub.3 Br consists in thermal bromination of trifluoromethane. In the absence of a catalyst, this reaction is generally carried out at temperatures from 600.degree. to 700.degree. C. so as to obtain a high conversion ratio.
The U.S. Pat. No. 3,456,024 relates to a process for preparation of perfluoroalkyl or perfluoroalkylene chlorides or bromides which consists in reacting, in a temperature range from 450.degree. to 700.degree. C., chlorine or bromine with various compounds containing a perfluoroalkyl R.sub.f or perfluoroalkylene R'.sub.f radical such as the compounds R.sub.f SF.sub.5, R.sub.f SO.sub.2 X, (R.sub.f SO.sub.2).sub.2 O, HR'.sub.f CH.sub.2 OH, R.sub.f H and R.sub.f (CH.sub.2 CH.sub.2).sub.n X', X designating a fluorine, chlorine or bromine atom or a hydroxyl group, X' a chlorine or bromine atom, and n a number from 1 to 5. In this Patent, bromination of a hydroperfluoroalkane R.sub.f H is only illustrated by a single example, namely Example 7 relating to the bromination of the compound n-C.sub.7 F.sub.15 H with the following C.sub.7 F.sub.15 Br yields by mole:
______________________________________ Temperature (.degree.C.) Yield ______________________________________ 500 29% 550 49.5% 600 81.4% ______________________________________
No mention is made of the selectivity of the reaction, which is a very important parameter for implementation of an industrial process.
Considering the high temperatures necessary to obtain industrially acceptable conversion ratios both in the bromination of trifluoromethane and in that of the compound n-C.sub.7 F.sub.15 H, the application of this technique to industrial preparation of PFOB for medical use from 1-hydroheptadecafluoro-n-octane (C.sub.8 F.sub.17 H) could not seriously be envisaged. In fact, this compound is thermally unstable and decomposes above 510.degree. C. into toxic products, in particular perfluoroisobutene (PFIB) which is known for its extreme toxicity (see for example the articles by E. W. Cook and J. S. Pierce, Nature, 242, 1973, p. 5396-7 and by J. W. Clayton, Environmental Health Perspectives 21, 1977, pp. 255-267). Moreover, PFOB itself is thermally unstable above 520.degree. C.
It has now been found that, in the particular case of the compound C.sub.8 F.sub.17 H and its brominated derviative, a high yield for the reaction can be obtained while operating at temperautres lower than those taught by the prior technique.
The process according to the invention for continuous preparation of PFOB is characterised in that bromine is reacted in the gaseous phase with 1-hydroheptadecafluoro-n-octane in a Br.sub.2 /C.sub.8 F.sub.17 H mole ratio between 0.2 and 5 (preferably between 0.5 and 2.5), at a temperature between 450.degree. and 520.degree. C. (preferably between 470.degree. and 510.degree. C.) and with a contact time between 2 and 240 seconds (preferably between 5 and 60 seconds).
This set of operating conditions leads to a high conversion ratio and a selectivity at least equal to 99%, without formation of toxic subproducts like PFIB. The unprocessed PFOB obtained is furthermore free from any trace (detection limit: .ltoreq.100ppm) of residual C.sub.8 F.sub.17 I which might be contained in the starting C.sub.8 F.sub.17 H.
The process according to the invention can be implemented in any kind of reactor which is suitable for a gaseous phase reaction, in particular in a tubular reactor optionally fitted with packing in order to promote mixture of the gases. Equipment made of quartz or of glass is advantageously used, but any metallic material can also be used (for example Monel, Inconel and Hastelloy) which is capable of resisting the corrosive action of bromine and hydrobromic acid at the reaction temperature chosen.
Although it is not indispensable, the reaction of bromine with C.sub.8 F.sub.17 H can be carried out in the presence of an inert gaseous diluent such as, for example, nitrogen or PFOB itself.
Industrially, it is preferred to work at atmospheric pressure, but working at a pressure greater than atmospheric pressure would not depart from the scope of the present invention, provided that the reaction system stayed in the gaseous state.
The gases leaving the reactor which contain the PFOB formed and the unconverted C.sub.8 F.sub.17 H, as well as the hydrobromic acid which is a sub-product of the reaction: EQU C.sub.8 F.sub.17 H+Br.sub.2 .fwdarw.C.sub.8 F.sub.17 Br+HBr
and possibly unreacted or excess bromine are cooled, then neutralized by an aqueous alkaline solution and/or reduced by an aqueous solution of sodium sulphite or metabisulphite. After pouring off the organic phase, the unconverted compound C.sub.8 F.sub.17 H can, after separation by distillation, be recycled to the entry of the reactor.
In accordance with an advantageous method, the gases leaving the reactor can, before neutralization and/or reduction, be treated by chlorine so as to reoxidize the hydrobromic acid which is a sub-product into bromine which can thus be recycled in the process.